This invention relates to an electronic device comprising a thin film transistor carried on a transparent, insulating, substrate and having spaced source and drain electrodes, an intrinsic amorphous silicon semiconductor layer extending between the source and drain for providing a channel region between the source and drain electrodes, and a gate insulator layer and gate electrode extending adjacent to the intrinsic semiconductor layer. The electronic device may be a flat panel display device, for example an active matrix liquid crystal display panel, or another type of large area electronic device such as a large area image sensor, a touch sensor or a thin film memory device.
There is presently considerable interest in developing thin film circuits comprising thin film transistors, hereinafter referred to as TFTs, on glass and other insulating substrates for large area electronics applications. The TFTs may form switching devices in an active matrix address array of elements, for example liquid crystal display elements and/or integrated drive circuits for such element matrices.
Examples of an active matrix liquid crystal display device using top gate amorphous silicon TFTs as switching devices for the display elements are described in EP-B-0217 406. In these devices, crossing sets of row and column address conductors are provided on a glass substrate together with a display element electrode adjacent each intersection between the row and column conductors which define a row and column array of display elements. Each display element electrode is connected to its associated row and column conductors via a top gate TFT also carried on the substrate. The amorphous silicon material forming the semiconductor channel region extending between the source and drain electrodes across an intervening part of the substrate surface has photoconductive properties and light for example from a backlight used to illuminate the display device through the substrate carrying the TFTs and incident on the channel region causes photocurrents due to light absorption in the amorphous silicon material. The resulting leakage currents can seriously affect the "off" state resistance of the TFT leading to significant charge leakage from the display element and degrading the quality of the display output. In order to reduce these effects, the channel region of each of the TFTs of the display device can be protected by an opaque light shield provided on the glass substrate and underlying the channel region, as described in the aforementioned specification, which serves to prevent light directed through the substrate towards the TFT from a backlight from reaching the channel region of the semiconductor layer. The light shields are provided as discrete islands of metal formed by patterning photolithographically a metal layer deposited over the substrate surface prior to forming the TFT structures. The light shields, and the intervening surface regions of the glass substrate, are covered by a protective insulating layer of, for example, silicon dioxide, that provides a planar surface on which the TFTs, address lines and display element electrodes are then formed, with the semiconductor layer of the TFTs then extending directly over the surface of this protective layer between the source and drain electrodes so as to be spaced, and insulated, from the underlying metal light shields. While the light shields adequately protect the semiconductor layers of the TFTs from exposure to illuminating light, their provision, and the provision of the necessary overlying layer of insulating material requires an additional photomask operation and two deposition steps in the fabrication process. Also, such metal light shields may cause unwanted parasitic capacitance effects.